Hazardous substances such as particulate matter (PM), nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like are present in the exhaust gas of diesel engines that are installed in vehicles such as trucks, buses and the like.
Measures taken in recent years to reduce emissions of such hazardous substances from vehicles include measures for reducing the generation of hazardous substances by improving combustion in the diesel engine, and measures for removing, as much as possible, the above-described hazardous substances in the exhaust gas, by arranging a plurality of post-processing units, for instance, an oxidation catalyst (DOC: Diesel Oxidation Catalyst), a diesel particulate filter (DPF: Diesel Particulate Filter), a urea-selective reduction catalyst (urea SCR: Urea-Selective Catalytic Reduction) or a NOx storage-reduction catalyst (LNT: Lean NOx Trap), in an exhaust passage of a diesel engine (see Japanese Patent Application Publication No. 2009-264147 and Japanese Patent Application Publication No. 2009-257226).